Devices formed from semiconductor materials can possess unique electromechanical properties imparted by the material. However, device performance can be negatively affected by techniques employed to etch the semiconductor material. For instance, some wet chemistry techniques provide isotropic etching, in which a high aspect ratio structure is difficult to attain. Dry chemistry techniques, such as those employing plasma, generally provide anisotropic etching in which etching occurs more favorably in one dimension. Thus, high aspect ratio structures can be more readily achieved using dry chemistry techniques. Yet additional processes are required, as some dry chemistry techniques can be optimized to provide smooth, etched surfaces but at a trade-off for etch rates, sidewall angles, etc.
III-V semiconductor materials have numerous device applications. When etching III-V materials containing aluminum, it is generally believed that BCl3 is required in order to remove and etch any aluminum oxides present in the material (see, e.g., U.S. Pat. No. 5,624,529). Thus, most processes require BCl3 throughout the entire etching process at particularly favorable percentages (see, e.g., U.S. Pat. No. 7,196,017). While such BCl3-containing processes can provide smooth sidewalls, vertical sidewalls (i.e., approaching an angle of 90°) are still difficult to attain. In some III-V devices, such as resonators, vertical sidewalls improve quality factors and reduce detrimental spurious modes. Thus, there is a need for additional processes to provide vertical sidewalls in semiconductor devices.